Electrostatic printing on metal substrates



United States Patent 3,401,037 ELECTROSTATIC PRINTING 0N METAL SUBSTRATES Jerome Roteman, Freehold, N.J., and Sebastian T. Brancato, Queens Village, N.Y., assignors to Interchemical Corporation, New York, N.Y., a corporation of Ohio No Drawing. Filed Nov. 25, 1964, Ser. No. 414,024 5 Claims. (Cl. 96-1) ABSTRACT OF THE DISCLOSURE An electrostatic printing process for developing a visible image on a metal substrate carrying a coating comprising a dispersion of photoconductive zinc oxide in a vehicle of an electrically insulated, air-dried, tack free, uncross-linked film of a thermosetting resin in a volatile organic solvent. The coated film is charged and the image formed in the conventional manner. The image is developed and the film simultaneously cured by exposure to heat.

This invention relates to electrostatic printing and particularly to electrostatic printing on metal substrates.

An electrostatic printing process is a process for producing a visibile image or copy by converting a light image or signal into an electrostatic charge pattern on an electrically insulating base. The charge pattern is then converted or developed into a visible image by the application thereto of electrically-attractable particles. A typical electrostatic printing process utilizing a photoconducting insulating stratum includes first producing a blanket electrostatic charge on the surface of the photoconducting stratum. The electrostatic charge may be stored on th surface for a time in the dark. Within the period in which a substantial charge remains, a light image is focused on the charged surface, discharging the portions of the surface irradiated with light, leaving the remainder of the surface in a charged condition, and thereby forming an electrostatic image thereon. The electrostatic image is rendered visible by applying to the electrostatic image a developer substance, such as a pigmented thermoplastic resin powder, which is held electrostatically to the charged areas of the surface. The visible image thus formed may be fixed directly to the surface, for example, by fusing the image thereto.

A process of electrostatic printing in which the thermoplastic resin powder is fused directly to the surface of the stratum is described in US. Patents 3,052,539 and 3,052,540, both to H. G. Greig.

Attempts have been made to utilize recording elements of the type described in the Greig patents in printing on tinplate and other metal substrates with less than fully desirable results. In such methods, photoconductive zinc oxide particles are usually dispersed in an electrically insulating thermoplastic binder and coated on the metal substrate. The electrostatic image is then formed and developed or printed, after which a clear thermosetting protective overcoating is applied. There have been some instances where thermosetting binders have been considered in place of the thermoplastic binders. However, these thermosetting binders have always been heated to effect'at leasta substantial portion of crosslinking or setting and usually a complete setting prior to the forming and printing of the image.

Irrespective of whether the binder is thermoplastic or thermosetting, we have found that when the el ctrostatic printing is done in the manner described above, a thermosetting protective overcoating must be applied or else the printed matter lacks abrasion resistance, being easily removed or marred by handling.

We have now discovered a novel method of electrostatic printing on metal substrates which eliminates the need for the thermosetting overcoating.

Our method involves providing the metal substrate with a coating comprising a dispersion of photoconductive Zinc oxide in an electrically insulating solution in a volatile organic solvent of an electrically insulating thermosetting resin which does not cross-link on air-drying. (By air-drying, is meant permitting the solvent to evap rate at the ambient temperature.) In other words, the resin must be one which requires heat in order to under.- go any degree of setting. Such resins are well known to those skilled in the art. While air drying is the preferred method of eliminating the solvent, the elimination of solvent may also be carried out at elevated temperatures if the temperature is sufliciently low enough to prevent any appreciable cross-linking of the resin.

The applied coating is air-dried to a tack-free, uncrosslinked film. The coated plate is then charged and eX- posed to the image in the conventional manner. The developer is applied conventionally. After the application of the developer, the developed coating is heated for the temperature and time required to set the particular thermosetting resin binder. The resulting coated panel has the printed or developed matter firmly fixed. The rub resistance is excellent. The panel may be bent and otherwise worked without affecting the printing.

The zinc oxide particles are conventional photoconductive zinc oxide particles of the type described in the aforementioned Greig patents.

The binder resins must be thermosetting and electrically insulating. That is they must have high dielectric strengths of the same order as those of the thermoplastic resins described in said Greig patents. In addition, the thermosetting resin must be one which while air-drying to a tack-free film does not undergo any cross-linking unless heat is applied. Classes of this type of thermosetting resin are well known in the art. They include ureaformaldehyde resins, triazine-formaldehyde resins, phenolic resins, epoxy resins, methylolated acrylamide resins, styrenated alkyds and m'ethacrylated alkyds.

During the printing of metals which are to be used in cans and containers, the coated panels are often piled in stacks prior to printing. Accordingly, it is preferable that the coatings have good block resistance.

Block resistance is determined by placing the coated surfaces of two panels face to face and applying a given pressure per unit area at a given temperature. If the surfaces do not stick together, then the coating is said to have a block resistance greater than the appled pressure at the temperature. When the panels are to be stacked during the printing process, it is preferable that the coatings have a block resistance greater than 1.5 lbs./ sq. in. at 25 C.

The developers used in this invention are preferably liquid developers. Conventional liquid developers e.g., the

liquid developers of Patent No. 2,907,674 which are' charged particles dispersed in an insulating liquid of high volume resistivity e.g., toluene, cyclohexane, n-pentane and carbon tetrachloride may be used. Other suitable liquid developers are those described in Patent No. 3,078,231.

While the preferred embodiments of this invention have been described in terms of liquid developers, it should be clear that the conventional powder developers may be used in place of the liquid developers. When powders are used the exposure and printing procedures of US. Patent No. 3,052,539 as well as the powder developers described in said patent may be used.

The following examples will further illustrate the practice of this invention. Unless otherwise set forth, all proportions in this specification and claims are by weight.

3 EXAMPLE 1- The following mixtureis ball milled to a smooth consistency:

Parts by weight A 48% solution in 1:1 xylene: butanol solvent of butoxy-methylolated acrylamide copolymer consisting of the reaction product of a 30% solution of formaldehyde in butanol with a copolymer of 75% styrene, acrylamide, 7% ethyl acrylate and 3% methacrylic acid; the formaldehyde solution: copolymer ratio is 1:3 Zinc oxide (photoconductive) A 0.12% solution in methanol of methylene blue dye 2.0

Then, sulficient xylene is added to reduce the viscosity of the mixture to 25 sec. on a Ford No. 4 Cup. The mixture is then sprayed by conventional apparatus on to tinplate panel and permitted to air-dry at the ambient temperature for about 2 to 3 hours. The resulting white coating is tack-free and has block resistance of about two pounds per square inch at room temperature. In order to print on the panel electrostatically, the coating on the panel is charged negatively in accordance with the procedure of US. Patent No. 2,907,674 and exposed to a positive transparency of the image to be printed. Then, following the procedure of said patent, a liquid developer of the following composition is applied to the plate:

A dispersion of 1 g. of peacock blue pigment in 9 g. of mineral oil is in turn dispersed in 10,000 ml. Isopar G solvent containing 7 g. of manganese naphthenate (aliphatic hydrocarbon solvent having a boiling range of 315-350 F. and a K. B. Value of 27.4.)

The developed image appears as blue printing on the white background. The panel is then baked at 350 F. for /2 hour to cure the thermosetting coating and to fuse the deposited developer to the coating. The coating is hard, tough and well cured, and the developer which is incorporated into the cured coating displays excellent ad hesion and rub-resistance.

EXAMPLE 2 Example 1 is repeated using the same procedure conditions, ingredients and proportions except that in the coating, the following formulation is used:

Parts by weight Methylon 75108 (Phenolic resin consisting of a mixture of the alkyl ethers of mono-, diand trimethylol phenol, the tri-component predominating, prepared in accordance with US. Patent No. 2,579,330, par- A 1:1 mixture of toluene and ethyl acetate in an amount sufficient to reduce the viscosity of the composition to 25 seconds on a Ford No. 4 Cup.

The results are substantially the same as in Example 1.

EXAMPLE 3 Example 1 is repeated using the same procedure, conditions, ingredients and proportions except that in the coating, the following formulation is used:

Parts by weight Epon 1007 (Epoxy resin, formed by the reaction of epichlorohydrin and Bisphenol A, having an average molecular weight of 2625, an epoxide equivalent weight of l5502200, a hydroxyl equivalent weight of 200 and 1.3 to 1.7 epoxides per molecule Urea-formaldehyde 30 Zinc oxide (photoconductive) 50 0.12% solution in methanol of methylene blue dye 2.0

and, A 1:1 mixture of xylene and methyl isobutyl ketone in an amount sufiicient to reduce the viscosity of the composition to 25 seconds on a Ford No. 4 Cup.

The results are substantially the same as in Example 1.

EXAMPLE 4 Example 1 is repeated using the same procedure, conditions, ingredients and proportions except that in the coating, the following formulation is used:

Parts by weight Styrenated 60% oil length phthalic alkyd resin having a 1:1, styrenezmodified alkyd content, the oil modifying the alkyd being a 1:1, soya-dehydrated castor oil blend Zinc oxide (photoconductive) 50 0.12% solution in methanol of methylene blue dye 2.0 and, xylene, in an amount sufficient to reduce the viscosity of the composition to 25 seconds on a Ford No. 4 Cup.

The results are substantially the same as in Example 1.

EXAMPLE 5 Example 1 is repeated using the same procedure, conditions, ingredients and proportions, except that in the coating, the following formulation is used:

Parts by weight A methacrylated 30% oil length alkyd resin, the methyl methacrylate content of which is 35% and the modifying oil of which is dehydrated castor oil 100 Zinc oxide (photoconductive) 50 0.12% solution in methanol of Methylene Blue dye 2.0 and, Xylene in an amount sufficient to reduce the viscosity of the composition to 25 seconds on a Ford No. 4 Cup.

The results are substantially the same as in Example 1.

In addition to tin-plate as described in the examples, the process of this invention may be used on a wide variety of metal substrates such as aluminum, copper, brass and steel plates.

While there have been described what is at present considered to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed to cover all such changes and modifications as fall within the true spirit and scope of the invention.

We claim:

1. A process for forming a visible image on metal substrates consisting of the steps of (A) coating a surface of the metal substrate with a composition comprising a dispersion of photo-conductive zinc oxide in an electrically insulating filmforming thermosetting resin, selected from the group consisting of urea-formaldehyde resins, triazineformaldehyde resins, phenolic resins, epoxy resins, methylolated acrylamide resins, styrenated alkyds and methacrylated alkyds, which does not cross-link on air-drying in a volatile organic solvent,

(B) air-drying the applied coating to a tack-free uncross-linked film,

(C) forming a pattern of electrostatic charges corresponding to said image on the surface of said coating,

(D) developing said pattern by depositing a finelydivided developer substance on the surface bearing said pattern in a configuration conforming with said pattern, and

5 (E) then simultaneously fixing the deposited developer and setting said thermosetting resinous film by heating at 350 F. for 30 minutes.

2. The process of claim 1, wherein the developer is a dispersion of finely-divided charged pigment particles in an electrically insulating liquid.

3. The process of claim 1, wherein the thermosetting resin is one which gives uncross-linked films upon airdrying having a block resistance greater than 1.5 1bs./sq. in. at 25 C.

4. The process of claim '1, wherein the resin is a copolymer comprising the polymerization product of 75% styrene, 15% acrylamide, 7% ethyl acrylate and 3% methacrylic acid.

plate.

6 References Cited UNITED STATES PATENTS Van Dorn et al 961.8 Kucera 961.8 Semegen 117-76 Middleton et al 96-15 Metcalfe et a1 961 Wolff u 96-l.8 Sciambi 961.5 Gray 961.5 Fox 96--1.5

NORMAN G. TORCHIN, Primary Examiner. 5. The proces of claim 1, wherein said metal is tin- 15 C. E, VAN HORN, Assistant Examiner. 

